Improvements in or relating to tempered glazings and glass for use therein

ABSTRACT

Glazing, thermally tempered to required standards, are produced more readily by tempering panes having a high coefficient of thermal expansion (greater than 93×10 −7  per degree Centigrade) and for a low Fracture Toughness (less than 0.72 MPam ½ ). Use of glasses selected according to the invention enables thin glazings (especially glazings less than 3 mm thick) to be tempered to automotive standard with improved yields using conventional tempering methods, and thicker glazings to be tempered at lower quench pressure than required hitherto. Suitable glasses include glasses comprising, in percentages by weight, 64 to 75% SiO 2 , 0 to 5% Al 2 O 3 , 0 to 5% B 2 O 3 , 9 to 16% alkaline earth metal oxide other than MgO, 0 to 2% MgO, 15 to 18% alkali mental oxide and at least 0.05% total iron (calculated as Fe 2 O 3 )

BACKGROUND OF THE INVENTION

The invention relates to glazings of tempered glass, especially, but notexclusively,

1. Field of the Invention

Glazings of thin tempered glass (normally tempered float glass) forautomotive use, a method of tempering a glazing, and to a novel glasscomposition suitable for use in the tempered glazings of the inventionand in the method of the invention.

2. Discussion of Related Art

Prior to the 1970's, automotive glazings were generally 4 mm or 5 mmthick or even thicker. The first oil crisis of the 1970's encouraged amove towards thinner glazings for automotive use, especially in Europeand Japan, and towards solving the problems encountered in producingthin tempered glazings having the fracture characteristics required tomeet official standards. In order to meet European standards, it wasfound necessary (because of the fracture characteristics of the glass)to provide a higher tempering stress together with an appropriate stressdistribution (see, for example, UK patents GB 1 512 163 and GB 2 000117) in order to achieve the required fracture patterns on breakage.Moreover, because of the reduced thickness of the glass, it was moredifficult to achieve the temperature differential between the surfaceand core of the glass required to produce a given tempering stress.While satisfactory tempering was achieved in thicknesses of about 3 mm,the difficulties of tempering thinner glasses by conventional processeshave inhibited progress in reducing glass thickness further so that,about 25 years after the introduction of such thin tempered automotiveglazings, the commercial production of tempered automotive glazingshaving a thickness of less than 3.1 mm remains difficult.

We have now found that glazings, especially but not exclusively thinnerglazings, can be more readily tempered including tempered to meetglazing standards (e.g. such as European automotive glazing standards)if the glass composition is appropriately modified, especially if theglass composition is modified to significantly increase its coefficientof thermal expansion and/or reduce its Fracture Toughness.

Certain selected glass compositions have previously been proposed foruse in thin automotive glazing. International Patent Application WO96/28394 relates to glass sheets of thickness in the range 2 to 3 mmhaving a total iron content (measured as Fe₂O₃) of 0.85 to 2% by weight,and specified optical properties, including a visible light transmissionof greater than 70% and a total energy transmission of less than 50%.The glasses specifically described have a high alkali metal oxidecontent (ranging from 14.4% to 15.8% by weight) a magnesium oxidecontent ranging from 0.25% to 3.8% by weight and a calcium oxide contentranging from 8.4% to 8.6% by weight. The specification refers to thepossibility of tempering single sheets of such thin glass for use inautomotive side glazings, but make no reference to the difficulty ofachieving commercially satisfactory tempering in practice.

International Patent Application WO 99/44952 relates to a sheet of sodalime silica glass designed to be heat tempered and characterised by avery high coefficient α of thermal expansion greater that 100×10⁻⁷ K⁻¹(although it does not specify the range of temperatures over which α isto be measured), a Young's Modulus E higher than 60 Gpa and a thermalconductivity K less than 0.9 Wm⁻¹K⁻¹. The invention is said to makepossible glass sheets of thickness lower than 2.5 mm which can betempered to the requirements of ECE Regulation R43 using apparatuspreviously envisaged for the tempering of 3.15 mm glass. The particularglasses described all have a very high alkali metal oxide content (inthe range 19.9 to 22.3% by weight) resulting in low durability andmaking the glasses expensive to produce.

SUMMARY OF THE INVENTION

According to the present invention there is provided a thermallytempered glazing of soda lime silica glass produced by tempering a paneof glass having a coefficient of thermal expansion, α, greater than93×10⁻⁷° C.⁻¹ and/or a Fracture Toughness, FT, of less than 0.72MPam^(½). The invention is especially, but not exclusively, applicableto tempered glass panes less than 3 mm thick and to the tempering ofsuch panes.

DESCRIPITION OF THE PREFERRED EMBODIMENTS

For the purpose of the present specification and claims, α is the valueof the coefficient of thermal expansion per degree Centigrade of theglass measured over the range 100° C. to 300° C.; it is measured inaccordance with ASTM E228 at constant heating rate. Preferably thecoefficient of thermal expansion is at least 95×10⁻⁷ per degreeCentigrade, although modification of the composition to achieve acoefficient of thermal expansion greater than or equal to 100×10⁻⁷,while beneficial to assist tempering, will generally be avoided on costand durability grounds.

Toughness is the energy per unit area (Joules per square metre) requiredto make a crack grow. Fracture Toughness, FT, is related to Young'smodulus and surface energy by

FT=(2×Surface Energy×Young's Modulus/1−v ²)^(½)

where v is Poisson's ratio. For the purpose of the present specificationand claims, it is determined by indenting a bar of glass using a Vickersindenter at a load sufficient to produce cracks at the corners of theindentation, and then breaking the bar in a 3- or 4-point bend test andthe determining fracture stress, σ_(f), in Pascals required forbreakage. The Fracture Toughness of the glass, assuming it is in thefully annealed state*, is then given by

FT=η(E/H)^(⅛)σ_(f) ^(¾) P ^(¼)

where η is a constant, E is Young's modulus, H is the hardness of theglass, and P is the load used to create the indentation.

Flat glass will be the fully annealed state if it has been heated at theannealing temperature for one hour and cooled at 2° C. per minute toroom temperature.

The constant η is determined with reference to FIG. 8.20 in Fracture ofBrittle Solids (Brian Lawn, Cambridge University Press 1993). Applyingvalues of E=70 GPa, H=5.5 GPa and FT=0.75 MPam^({fraction (1/2 )})forsoda lime silica glass the value of η is η=0.44.

If the glass is not in the fully annealed state*, it is necessary toapply a correction for residual stress to the Fracture Toughnesscalculated using the above equation. In practice, it is convenient tomeasure the Fracture Toughness of glass in the fully annealed state.

Preferably, the glass has a Fracture Toughness of less than or equal to0.70 MPam^(½), especially less than or equal to 0.68 MPam^(½).

In a preferred embodiment of the invention, the glass has a coefficientof thermal expansion α (° C.⁻¹ in the range 100° to 300° C.) and aFracture Toughness, FT (in MPam^(½)) such that$\frac{\alpha \times 10^{7}}{FT} \geq 135$

preferably ≧140, and especially ≧145.

It has been found that an increase in the alkali metal oxide content ofthe glass tends to increase the coefficient of thermal expansion, andwhile it is well known that glass can be produced with high alkali metalcontents (patents relating to glass compositions for production by thefloat process typically propose an alkali metal oxide content in a rangeup to about 20%), an increase in alkali metal oxide content generallyincreases the cost of the glass and reduces its durability. Inconsequence, commercially available float glass generally has an alkalimetal oxide content in the range of 13 to 14% by weight, and glasseswith higher alkali metal oxide contents are not used in the productionof thermally tempered glazings, especially automotive glazings. We havefound that increasing the alkali metal oxide content by a relativelysmall amount results in a surprising increase in the ease of tempering(as measured, for example, by the particle count on fracture) of theglass (especially when associated with a modification of the alkalineearth metal oxide content of the base glass as explained below). Thuscertain preferred glasses have an alkali metal content greater than 15%by weight, preferably less than 19% (to avoid excessive cost and loss ofdurability) and especially in the range 15% to 18% by weight, especiallypreferred glasses have an alkali metal content of 15% to 17% by weight.The sodium oxide content is preferably greater than 14.5% by weight.

Further improvements in ease of tempering appear to result fromincreasing the ferrous oxide content of the glass, and we especiallyprefer to use glass compositions containing at least 0.2%, especially atleast 0.3%, by weight of ferrous oxide (calculated as ferric oxide),and, in one embodiment of the invention, that at least 30% (preferablyat least 35%) of any iron oxide present to be in the form of ferrousoxide (where, in calculating the percentages, both ferric oxide andferrous oxide are calculated as if ferric oxide).

The alkali metal oxide is believed to operate both by increasing thecoefficient of thermal expansion of the glass (so increasing the stressdifferential between the surface layers of the glass and the coreresulting from a given temperature difference between surface and core)and reducing the thermal conductivity of the glass (so increasing thetemperature differential between surface and core when the surface israpidly cooled in a thermal tempering process). However, the resultsachieved, especially with glasses containing significant amounts offerrous iron, show a much greater increase in ease of tempering to meetEuropean automotive glazing standards than can be accounted for by theseeffects alone, and these can be attributed, at least in part, to areduction in Fracture Toughness of the glass.

One effect of increasing the alkali metal oxide content in asoda-lime-silica glass is believed to be an increase in the proportionof non-bridging oxygens (a bridging oxygen being an oxygen bondeddirectly to two silicon atoms. Si—O—Si) present:

≡Si—O—Si+Na₂O→≡Si—O—Na+Na—O—Si≡

The formation of such non-bridging oxygens in a silica lattice leads toa weakening of the glass structure, which is associated with reducedFracture Toughness, and we have found that the reduced FractureToughness is associated with increased ease of tempering. The effect ofincorporating alkaline earth metal ions in a silica lattice is similarlyto displace oxygens directly bridging between silica atoms:

≡Si—O—Si≡+MO→≡Si—O—M—O—Si≡

where M is an alkaline earth metal. Differences in bonding strengthsoccur through different sizes of alkaline earth metal ions. In general,we believe the smaller the alkaline earth metal incorporated, thestronger the lattice and the higher the Fracture Toughness of the glass,with the difference between calcium ions and magnesium ions beingparticularly marked. Thus, to decrease the Fracture Toughness of theglass, it is desirable to maintain the magnesium content of the glasslow (less than 2%, preferably less than 1%, especially less than 0.5%,all by weight), while avoiding use of an excessively high (from a costviewpoint) proportion of alkali metal oxide will generally imply acontent of alkaline earth metal oxide, other than magnesium oxide, of atleast 9% and preferably at least 10%, by weight. Preferably, the glasswill contain at least 9% and especially at least 10% of calcium oxide,and the total alkaline earth metal oxide content (including magnesiumoxide) of the glass will normally be more than 10% by weight.

The glass will usually be float glass with a composition (in percentagesby weight) of:

SiO₂ 64-75% Al₂O₃  0-5% B₂O₃  0-5% Alkaline earth metal oxide  6-15%(alkaline earth metal oxide other than MgO preferably 9-15%) Alkalimetal oxide 15-20% (preferably 15-17% with sodium oxide, preferably morethan 14.5% especially more than 14.75%) Total iron (calculated as Fe₂O₃)preferably greater than 0.3%, especially 0.5-2.5% TiO₂  0-1%

Certain of the glass compositions which may be used in the practice ofthe present invention are new, and according to a further aspect of theinvention, there is provided a novel soda lime silica glass in sheetform of composition comprising in percentage by weight:

SiO₂ 64-75% Al₂O₃  0-5% B₂O₃  0-5% Alkaline earth metal oxide (otherthan MgO)  9-16%, preferably 10-16% MgO <2% Alkali metal oxide 15-18%Total iron (calculated as Fe₂O₃) ≧0.05%

and any small proportions of additional components, for example, titaniaand other colouring agents, for example, selenium, cobalt oxide, nickeloxide, chromium oxide, cerium oxide.

Preferably the glass composition contains, in percentages by weight,67-73% SiO₂, 0-3% Al₂O₃, 0-3% B₂O₃, alkaline earth metal oxide (otherthan MgO) 10-14%, alkali metal oxide 15-17%.

While magnesium oxide contents below 0.5% may be preferred for optimumresults, in practice, achieving a very low magnesium content willgenerally imply a long changeover time when the glass is madesuccessively with a conventional glass containing a higher proportion(typically around 4%) of magnesium oxide and, in practice therefore, wenormally prefer to employ glasses containing at least 0.5% by weight ofmagnesium oxide. Moreover, for such practical reasons, a magnesium oxidecontent in the range 0.75% to 1.5% by weight will commonly be preferred.

The novel glasses of the present invention will normally contain iron,either to modify the optical properties and/or enhance the temperabilityof the glass, or at least as an impurity (since the use of iron freebatch materials is likely to add significantly to the cost of thebatch); in the latter case it will normally be present in an amount ofat least 0.05% by weight (calculated as ferric oxide).

In the former case, iron will normally be present in an amount(calculated as ferric oxide) of at least 0.5% by weight. For aparticularly high performance i.e. high visible light transmission withrelatively low solar energy transmission, the percentage of iron in theferrous state will be above 30%. In other cases, the percentage of ironin the ferrous state will be less than 30% (i.e. the ratio of ferrousiron (calculated as ferric oxide) to total iron (calculated as ferricoxide) in the glass will be less than 30%).

Preferred ranges of compositions are as discussed above in relation tothe tempered glazing of the invention. These glasses are used in sheetform and will normally have a thickness in the range 1 to 6 mm,especially 2 to 5 mm, and be formed by the float process.

A particularly preferred glass according to the present invention hasthe following composition in percentages by weight:

SiO₂ 71.0 CaO 10.5 Fe₂O₃ 1.0 Al₂O₃ 1.11 MgO 0.21 Na₂O 14.9 K₂O 0.64 TiO₂0.35 SO₃ 0.17 % Ferrous 35

which composition is hereinafter referred to as Composition I.Composition I has a coefficient of thermal expansion, α, of 98.9×10⁻⁷°C.⁻¹ (in the range 100° C. to 300° C.), and a Fracture Toughness of0.66±0.02 MPam^(½), so that, for Composition I:$\frac{\alpha \times 10^{7}}{FT} = {\frac{98.9}{0.66} = 150}$

The use of the specially selected glass compositions in accordance withthe present invention facilitates the production of thin (less than 3mm) tempered glasses, and is especially valuable in permitting thecommercial production of tempered automotive glazings in thicknesses of2.3 to 3 mm, especially 2.6 to 2.9 mm, by conventional temperingmethods. It is known that glazings below 3 mm can be tempered usingspecialist tempering processes, such as powder tempering, or specialtempering boxes available in commerce from Glasstech Inc of Perrysburg,Ohio, USA; it is the ability to temper the glass by conventional methodswith satisfactory yields without additional cost that is especiallyvaluable. The glazings may be tempered to meet national andinternational (especially European Standard ECE R43) standards forautomotive glazings, especially sidelights and backlights.

Even thinner glasses, for example glasses having a thickness in therange 1.0 mm to 2.5 mm, especially 1.6 mm to 1.9 mm, can besemi-tempered e.g. tempered to semi-dicing fracture, e.g a surfacecompressive stress of at least 35 MPa, in accordance with the inventionfor use in laminated automotive glazings (especially opening sideglazings required to pass a door slam test).

While the main advantage of using the special glass compositions of theinvention lies in tempering thin glasses by conventional methods, theiruse in thicker glasses is also valuable in enabling the requiredstresses to be achieved with lower heat transfer coefficients and hencelower blowing pressures, with a consequent reduction in the use ofenergy.

Thus, according to a further aspect of the present invention there isprovided a method of tempering a glazing (especially an automotiveglazing) composed of glass having a high (greater than 93×10⁻⁷° C.⁻¹)coefficient of thermal expansion and/or a low Fracture Toughness (lessthan 0.72 MPam^(½)) by operating at quench pressure at least 10% less,normally more than 20% less and preferably at least 25% less, than thequench pressure required to temper a corresponding glazing of standardcomposition to the required standards. Under optimum circumstances, useof the present invention may permit achievement of required temperingstandards at a quench pressure 40% or more less than the quench pressurerequired to toughen a corresponding glazing of standard composition tothose standards. The required tempering standards vary from country tocountry but generally require achievement of dicing fracture. By“required standards” we mean the standards required by the authoritiesin the country in which the glazing is to be used. In Europe, this willgenerally be ECE R43 for automotive glazings.

The method of the present invention is especially applicable to glazingshaving a thickness in the range 3 mm to 5 mm glazings and will generallyresult in the use of blowing pressures of not more than 12.5 kPa (50inches water gauge), preferably not more than 10 kPa (40 inches watergauge), especially not more than 7.5 kPa (30 inches water gauge) for 3mm glass, to not more than 7.5 MPa (30 inches water gauge), preferablynot more than 6 kPa (24 inches water gauge) for 4 mm glazing, and notmore than 6 kPa (24 inches water gauge) preferably not more than 5 kPa(20 inches water gauge), for 5 mm glass. The values for blowing pressurequoted above are generally applicable with dwell times (the timesbetween the leading edge of the glass exiting the heated zone and thetrailing edge of the glass entering the quench) of around 5 or 6seconds; it will be appreciated, however, that the lower the dwell time(for a given temperature at exit from the heating zone), the lower theblowing pressure required.

The method of the present invention offers a number of advantages. Theuse of lower quench pressure results in a saving of energy and reducesthe risk of a visible orange peel effect on tempering. Moreover, since alower quench pressure may be used, equipment (especially air blowers)and conditions may be used to temper glazings of the selected glasscompositions of the invention which are thinner than the conventionalglazings which can be satisfactorily tempered using that equipment andthose conditions; thus, for example, equipment and conditions capable oftempering glazings of conventional composition having a of at leastthickness 5 mm may be used to toughen glazings of glass having amodified composition as taught herein of lesser thickness e.g. 4 mm.

The expression “standard composition” is used herein to refer to a knowniron containing glass used extensively for production of tempered 3.1 mmautomotive glazings and having the following composition in percentagesby weight:

SiO₂ 72.1% CaO 8.15% Fe₂O₃ 1.07% Al₂O₃ 0.52% MgO 3.96% Na₂O 13.7% K₂O0.28% TiO₂ 0.04% SO₃ 0.14% % Ferrous 25

The glass has a coefficient of thermal expansion, α, of 92.4×10⁻⁷ (inthe range 100° to 300° C.), and a Fracture Toughness of 0.71 MPam^(½),so that, for this glass,$\frac{\alpha \times 10^{7}}{FT} = {\frac{92.4}{0.71} = 130}$

Samples of the glass, referred to as OPTIKOOL™ 371, are available fromGroup Intellectual Property Department, Pilkington plc, St Helens.England.

The invention is illustrated but not limited by the following exampleswhich describe the thermal tempering of automotive side glazings andcomponents therefor in accordance with the invention.

EXAMPLE 1

Blanks for a front door glass for a typical family size saloon car werecut to size from float glass of Composition I and thickness 2.85 mm andprepared for bending and tempering by edge grinding and washing inconventional manner.

The blanks were loaded in turn into a horizontal roller furnace andheated in the furnace to a temperature in the range 650° C. to 670° C.Each blank was removed from the furnace on rollers and advanced into abending zone, where the rollers were lowered to deposit the glass blankon a peripheral female mould of appropriate curvature for the glazingrequired. The glass sagged on the mould under the influence of gravityto assume the required curvature. The mould carrying the curved glasswas then advanced between quench boxes where the glass was quenched withcold air at a pressure in the range 8 kPa (32″ water gauge) to 24 kPa(96″ water gauge). The mould carrying the curved tempered glazing wasremoved from the quench, the glazing allowed to cool to room temperatureand assessed for shape (fit to fixture), optical quality, surfacecompressive stress measured by differential stress refractometry (DSR)and behaviour on fracture at a central position. In each case, the shapeand optical quality conformed both to relevant ECE Standards and normalOE customer requirements.

The key parameters of the bending and tempering processes, and themeasured surface stress and fracture behaviour (expressed as the minimumand maximum number of particles observed in a 5 cm square on the surfaceof the glazing after fracture at a central position) are shown in theaccompanying Table 1.

The procedure described above was repeated using glass of composition Iof thickness 3.1 mm, and thereafter using OPTIKOOL™ 371 glass (standardcomposition as set out above) of thickness 3.1 mm. In all cases, theshape and optical quality conformed both to relevant ECE Standards andnormal OE customer requirements. Again, the key parameters of thebending and tempering processes, and the measured surface stress andfracture behaviour, are shown in the accompanying Table 1.

TABLE 1 Saloon front door glazing (sag bent and tempered) Glass exit¹Quench entry Quench pressure kPa Surface Fracture Thickness temperatureDraw² time, temperature, (inches water gauge) compressive pattern SampleComposition mm ° C. seconds ° C. upper/lower stress MPa Min/Max 1 I 2.89665 3.7 — 8/7 (32/28) 90 22/151 2 I 2.86 665 3.7 — 8/7 (32/28) 97 15/1443 I 2.85 664 5.7 597 8/7 (32/28) 83 1/23 4 I 2.85 664 5.7 598 8/7(32/28) 82 3/25 5 I 2.86 663 7.6 582 8/7 (32/28) 63 6 I 2.84 663 7.6 5848/7 (32/28) 65 1/6  7 I 2.89 665 3.7 610 17/16 (67/64) 101 137/379  8 I2.89 664 3.7 610 17/16 (67/64) 90 143/370  9 I 2.89 667 5.7 600 17/16(67/64) 100 58/237 10 I 2.89 664 5.7 598 17/16 (67/64) 81 55/173 11 I2.89 667 7.6 589 17/16 (67/64) 69 6/91 12 I 2.89 667 7.6 587 17/16(67/64) 71 8/70 13 I 2.89 666 3.7 613 22/21 (86/84) 93 165/407  14 I2.90 664 3.7 610 22/21 (86/84) 99 161/387  15 I 3.15 661 3.7 610 8/7(32/28) 91 63/198 16 I 3.16 662 3.7 — 8/7 (32/28) 97 67/254 17 I 3.15660 5.7 — 8/7 (32/28) 75 31/136 18 I 3.15 661 5.7 598 8/7 (32/28) 8337/131 19 I 3.15 660 7.6 583 8/7 (32/28) 73 4/6  20 I 3.15 659 7.6 5838/7 (32/28) 70 1/51 21 I 3.15 665 3.6 613 17/16 (67/64) 108 218/405  22I 3.15 664 3.6 610 17/16 (67/64) 107 219/456  23 I 3.14 668 5.7 59917/16 (67/64) 101 126/404  24 I 3.14 664 5.7 601 17/16 (67/64) 96123/341  25 I 3.14 665 7.6 587 17/16 (67/64) 69 49/251 26 I 3.14 667 7.6589 17/16 (67/64) 72 47/265 27 I 3.16 661 3.7 — 22/21 (86/84) 101217/498  28 I 3.16 661 3.7 — 22/21 (86/84) 103 265/482  29 OPTIKOOL ™371 3.14 662 3.7 610 8/7 (32/28) 77 17/125 30 OPTIKOOL ™ 371 3.13 6623.7 609 8/7 (32/28) 76 28/110 31 OPTIKOOL ™ 371 3.15 661 5.7 — 8/7(32(28) 70 11/45  32 OPTIKOOL ™ 371 3.15 665 5.7 — 8/7 (32/28) 88 8/5233 OPTIKOOL ™ 371 3.14 658 7.6 580 8/7 (32/28) 6 1/6  34 OPTIKOOL ™ 3713.14 658 7.6 581 8/7 (32/28) — — 35 OPTIKOOL ™ 371 3.13 669 3.8 61317/16 (67/64) 102 93/243 36 OPTIKOOL ™ 371 3.13 668 5.7 600 17/16(67/64) 91 46/108 37 OPTIKOOL ™ 371 3.13 666 7.6 587 17/16 (67/64) 7613/101 38 OPTIKOOL ™ 371 3.15 663 3.7 612 22/21 (86/84) 97 153/317  39OPTIKOOL ™ 371 3.15 661 3.7 — 22/21 (86/84) 97 131/376  ¹on exit fromthe furnace ²time between leading edge of the glass exiting the heatedzone and trailing edge of the glass entering the quench.

When the results (surface compressive stress and fracture pattern)obtained with samples 15 to 28 are compared with the results obtainedwith samples 29 to 39, the enhanced temperability of the ‘high’ alkalimetal oxide glasses used in accordance with the invention is apparent.Thus, with the same draw time (3.7 seconds) and quench pressure (8/7kpa) samples 15 and 16 of glass Composition I exhibited surfacecompressive stress of 91 MPa and 97 MPa respectively and fracturepatterns with 63/198 and 67/254 particles (in accordance with ECE R43),while samples 29 and 30 of OPTIKOOL™ 371 glass exhibited surfacecompressive stresses of 77 MPa and 76 MPa respectively, with fracturepatterns exhibiting 17/125 and 28/110 (failing to meet ECE R43, thereduced number of particles corresponding to the reduced compressivestress). In fact, the only samples of OPTIKOOL™ 371 tempered which meetECE R43 (a particle count between a minimum of 40 and a maximum of 450for glass less than 4 mm thick) are 35, 36, 38 and 39, all of whichemploy a minimum quench entry temperature of 600° C. and/or a quenchpressure of 17/16 kPa or higher. In contrast, the use of the higheralkali metal oxide glass composition in accordance with the inventionenables the standards to be achieved using lower pressures (samples 15and 16), or the same pressure with a lower quench entry temperature(samples 23 to 26), making possible a significant saving in energyconsumption.

EXAMPLE 2

Blanks for a front door glazing for a typical family size saloon werecut to size from glass of composition I and 2.6 mm thickness andprepared for tempering by edge grinding and washing in conventionalmanner.

The blanks were loaded into a horizontal roller furnace where they wereheated to 580° C. and advanced onto a gas hearth furnace, in which theglasses were supported on a cushion of air from a bed shaped to therequired curvature. The glasses were heated to a temperature in therange 620° C. to 670° C. as they are advanced along the gas hearth andsagged to the required shape; after bending they were advanced into ahorizontal quench section where they were quenched between quenchnozzles above and below, while being supported by the quench air frombelow. The glasses were then removed from the quench, cooled to roomtemperature, and assessed for shape (fit to fixture), optical quality,surface compressive stress measured by DSR and behaviour on fracture ata central position.

The key parameters of the process, and the measured surface stress andfracture behaviour (expressed as minimum and maximum number of particlesobserved in a 5 cm square on the surface of the glazing after fractureat a central position) are shown in the accompanying Table 2. For eachsample, the shape and optical quality conformed both to relevant ECEstandards and normal customer requirements.

The results show that satisfactory tempering stresses were achievedusing modest quench pressures. While in each case, the requirements ofECE R43 for minimum and maximum number of particles were met, samples 2and 3 exhibited a number of splines (elongated glass particles longerthan 5 cm) where pressure would have led to failure to meet thatstandard. However, these could be avoided by inclusion of an additional“striping” nozzle in the quench in known manner (see, for example, UKpatent specification GB 2,000,117).

EXAMPLE 3

Blanks for a laminated front door glazing for a typical family sizesaloon were cut from glass of composition I of 1.8 mm thickness, andfrom OPTIKOOL™ 371 glass of 1.8 mm thickness and prepared for temperingby edge grinding and washing in conventional manner.

TABLE 2 Saloon front door glazing (bent and tempered on a gas hearth)Gas hearth furnace exit/quench Quench pressure Surface entry kPa (incheswater compressive Fracture temperature gauge) stress pattern Sample ° C.upper/lower MPa Min Max 1 665 20/11 (80/45) 87.3 44 224 2 650 20/11(80/45) 85.8 70 238 3 640 21/20 (84/80) 95.7 59 304

TABLE 3 Semi-tempered component of laminated front glazing for saloon(bent and tempered on gas hearth) Gas hearth Quench furnace pressureexit/quench entry kPa (inches Surface temperature water gauge)compressive Sample Composition ° C. upper/lower stress, MPa 1 I 633(11/8) 45/30 73.7 2 I 655  (6/5) 24/20 90.8 3 OPTIKOOL ™ 624 (11/8)45/30 61.1 371 4 OPTIKOOL ™ 645  (6/5) 24/20 57.4 371

The glass blanks were then bent and tempered on a gas hearth furnace andassessed as described in Example 2, except that, since they were to beused as the components of a laminated glazing, no fracture tests werecarried out.

The key parameters of the process and results of the stress measurementsare shown in the accompanying Table 3. Comparison between the samples 1and 2 of high alkali glass and samples 3 and 4, of standard glass showsthe increased stress achieved with the higher alkali glass in accordancewith the invention. While some of this increase may be due to the highertemperature of samples 1 and 2 at quench entry (for the same quenchpressures), this factor does not adequately explain the differenceswhich are attributable to the different composition of the glasses.

EXAMPLES 4 to 6

The following Examples describe production of samples of tempered glassand illustrate the improved ease of tempering resulting from appropriatechoice of glass composition to increase its coefficient of thermalexpansion while reducing its Fracture Toughness.

Samples of each of the glasses shown in accompanying Table 4 were meltedin the laboratory and cast into plates which were fully annealed. Thecoefficient of thermal expansion of each of the glasses was measured,together with its Fracture Toughness and, for the Comparative Exampleand Example 4, the centre tension. For Fracture Toughness, 20 bars ofeach glass were cut and polished to a normal size of 65×10×3.15 mm,measured as described above and the results averaged. To assess the easeof tempering of the sample glasses, a minimum 4 samples of each glass100×100×4 mm, all polished and edge worked, were tempered by heating at700° C. for 200 seconds on an oscillating horizontal furnace and thenquenched horizontally using the quench pressures shown in Table 4; thequench time was 155 seconds including cooling. The samples were thenfractured at the edge, and the number of particles formed in a square of5 cm side in the centre of the sample were counted and the resultsreported in the Table.

The glass used for the Comparative Example had approximately the samecomposition as OPTIKOOL™, modified by the omission of iron and theadjustment of other components present to compensate. Example 4 differedfrom the Comparative Example solely in the reduction of the MgO contentfrom 3.9 weight per cent to 0.1 weight per cent and its replacement withcalcium oxide. This adjustment has led to an increase in the coefficientof thermal expansion, from 91.4×10⁻⁷ per degree Centigrade to 93.9×10⁻⁷per degree Centigrade, and a reduction in Fracture Toughness from 0.70MPam^(½)to 0.67 MPam^(½). On tempering under identical conditions, theglass of Example 4 exhibited a higher centre tension (69.0 MPa) than theglass of the Comparative example (67.2 MPa), and a significantly higherparticle count on fracture (average 422, as compared to an average of374 obtained with the Comparative Example). It can thus be seen that thereduction in content of magnesium oxide and its replacement with calciumoxide has led to a significant improvement in the ease with which theglass may be tempered to a required standard. The ability to control theease with which the glass may be tempered (temperability) may beexploited in a number of ways, for example, to enable thinner glasspanes than hitherto to be satisfactorily tempered under given temperingconditions, or by reducing the severity of the conditions (withconsequent economies in running costs and, in appropriate circumstances,capital costs of the tempering operation) used for tempering.

Examples 5 and 6 similarly show the beneficial effect on tempering, asdetermined form the fractured patterns of the fractured glasses,resulting from replacing magnesium oxide (in Example 6) by calcium oxide(in Example 5), but in this case in a glass containing about 1% byweight of iron oxide and a higher alkali metal oxide content (above 15%by weight).

In these Examples, the tempering operation was carried out under lessstringent conditions than used in the Comparative Example and in Example4, so that, despite the higher alkali metal oxide content and morefavourable ratios of α×107 : Fracture Toughness, lower temperingstresses (and corresponding lower particle count on fracture) wereachieved on tempering to provide a fracture pattern in accordance withEuropean standards.

The difference in particle count on fracture between Examples 5 and 6 isattributed to the reduced magnesium oxide content and increased calciumoxide content of Example 5 compared to Example 6, which more thancompensated for the marginally higher alkali metal oxide content inExample 6, resulting in increased ease of tempering.

TABLE 4 Comparative Example Example Example Example 4 5 6 Oxide (weight%) SiO₂ 73.27 73.27 71.8 70.9 Na₂O 13.70 13.70 14.9 15.3 K₂O 0.33 0.330.3 0.3 MgO 3.91 0.10 1.1 3.9 CaO 7.92 11.73 10.1 7.8 Al₂O₃ 0.64 0.640.6 0.6 TiO₂ 0.05 0.05 0.04 0.04 ZrO₂ 0.04 0.04 Fe₂O₃ 0.92 0.92 SO₃ 0.300.30 0.25 0.25 % Fe in ferrous state 25% 25% α (100-300° C.) × 91.4 93.997.7 98.2 10⁷/ ° C. Fracture Toughness MPam^(1/2) with indentation loadof  2.94 N 0.71 0.64 0.61 0.75  4.91 N 0.66 0.63 0.67 0.68  9.81 N 0.680.69 0.63 0.68 19.62 N 0.74 0.72 0.67 0.72 Average 0.70 0.67 0.65 0.71 α× 10⁷ 131 140 150 138  FT Centre tension (MPa) 67.2 ± 1.5 69.0 ± 1.7Quench pressure kpa (psi) Upper 138 (20) 138 (20) 28 (4) 28 (4) Lower 69 (10)  69 (10) 14 (2) 14 (2) Particle count on fracture Example 1 344415 133 66 Example 2 375 476 123 81 Example 3 384 350 108 80 Example 4393 448 89 96 Example 5 114 68 Average 374 422 113 78

According to a modified aspect of the invention, a thermally temperedglazing of soda lime silica glass having a thickness of less than 3 mmis of green glass containing at least 14.5% by weight Na₂O, at least10.5% by weight CaO, at least 0.5% by weight total iron (measured asFe₂O₃) and being substantially magnesium-free. While, especially in thismodified aspect of the invention, the magnesium content of the glass isvery low, at least some magnesium is likely to be present as an impurityor a trace element in the batch or as a carry over from a previous runon the furnace; however, the maximum amount of magnesium present in thecomposition is unlikely to exceed about 0.2% by weight.

What is claimed is:
 1. A thermally tempered glazing of soda lime silica glass produced by tempering a pane of glass having an alkali metal oxide content in the range 15 to 18% by weight, a coefficient of thermal expansion greater than 93×10⁻⁷ per degree Centigrade and a Fracture Toughness of less than 0.72 MPam^(½).
 2. A thermally tempered glazing as claimed in claim 1 having a thickness of less than 3 mm.
 3. A thermally tempered glazing as claimed in claim 1 wherein the glass has a coefficient of thermal expansion of at least 95×10⁻⁷ per degree Centigrade.
 4. Thermally tempered glazing as claimed in claim 1 wherein the glass has a Fracture Toughness of less than 0.70 MPam^(½).
 5. A thermally tempered glazing as claimed in claim 1 wherein the coefficient of thermal expansion a per degree Centigrade and Fracture Toughness FT (in MPam^(½)) of the glass are such that $\frac{\alpha \times 10^{7}}{FT} \geq 135.$


6. A thermally tempered glazing as claimed in claim 1 wherein the glass has a ferrous oxide content (calculated as ferric oxide) of at least 0.2% by weight.
 7. A thermally tempered glazing as claimed in claim 6 wherein the glass has a ferrous oxide content (calculated as ferric oxide) of at least 0.3% by weight.
 8. A thermally tempered glazing as claimed in claim 1 wherein the glass has a magnesium oxide content of less than 2% by weight.
 9. A thermally tempered glazing as claimed in claim 1 wherein the glass has a content of alkaline earth metal oxide (other than magnesium oxide) of at least 9% by weight.
 10. A thermally tempered glazing as claimed in claim 1 having a thickness in the range 2.3 to 2.9 mm.
 11. A laminated automotive glazing comprising at least one semi-tempered glass pane having a thickness in the range 1.5 mm to 2.5 mm, produced by semi-tempering a pane of glass as claimed in claim
 1. 12. A thermally tempered glazing of soda lime silica glass having a thickness of less than 3 mm, the glass being green glass containing at least 14.5% by weight Na₂O, at least 10.5% by weight CaO, at least 0.5% by weight total iron (measured as Fe₂O₃) and being substantially magnesium-free, the glass having a ferrous value (% ferrous) of at least 30%.
 13. A glazing as claimed in claim 12, wherein the glazing is a thermally tempered glazing of soda lime silica glass produced by tempering a pane of glass having a coefficient of thermal expansion greater than 93×10⁻⁷ per degree Centigrade and a Fracture Toughness of less than 0.72 MPam^(½).
 14. A laminated automotive comprising at least one semi tempered glass pane having a thickness in the range 1.5 mm to 2.5 mm, and being of green glass having a composition comprising at least 14.5% by weight Na₂O, at least 10.5% by weight CaO, at least 0.5% by weight total iron (measured as Fe₂O₃) and being substantially magnesium-free, the glass having a ferrous value (% ferrous) of at least 30%.
 15. A laminated glazing as claimed in claim 14, wherein the glazing is a laminated automotive glazing comprising at least one semi-tempered glass pane having a thickness in the range 1.5 mm to 2.5 mm, produced by semi-tempering a pane of glass having a coefficient of thermal expansion greater than 93×10⁻⁷ per degree Centigrade and a Fracture Toughness of less than 0.72 MPam^(½). 